LM3101 datasheet(17/20 Pages) NSC

17 / 20 page

Application Hints

Pulse Feedback Section

During steady-state operation the LM3101 delivers pulse-

width modulated signals to the feedback circuit The feed-

back circuit will convert that signal into a series of AC-cou-

pled pulse signals and apply them to the LM3001 via the

pulse transformer (the first positive-edged pulse from the

LM3101 will cause the LM3001 to disconnect its internal

oscillator from its PWM and Output Driver circuits) The

feedback pulses will trigger the LM3001 Output Driver to

apply PWM drive signals to the Power MOSFET gate The

timing diagram in

Figure 12 demonstrates the feedback

communication

Pulse Interface Circuit

The pulse interface circuit provides isolation for the feed-

back circuit of the Offline Flyback Regulator The differentia-

tor circuit converts the PWM waveform into a pulse train

The differentiator delivers a train of 1VPK 15 ns wide pulses

to the pulse transformer The core should have high perme-

ability (typically 10000) at the switching frequency to allow

the transfer of energy with a very small transformer (size)

This one-to-one transformer transfers the pulse train to the

LM3001 via a 200X resistor which is used mainly to filter

noise from the system

TLH11436 – 18

FIGURE 12 Pulse Feedback Timing Diagram

TLH11436 – 19

FIGURE 13 Pulse Interface Circuit

LM3101 Datasheet(PDF) 17 Page - National Semiconductor (TI)